Exhaust emission control device for internal combustion engine

ABSTRACT

An exhaust emission control device for an internal combustion engine includes a capturing device that is disposed in an exhaust passage of the engine and that captures particulate matter in exhaust gas flowing through the exhaust passage, a sensor that is disposed in the exhaust passage on a downstream side of the capturing device in a flow direction of exhaust gas and that measures an amount of soot in the particulate matter, a control unit for increasing the particulate matter that flows into the capturing device, and a determination device for determining whether the capturing device is at fault based on the amount of soot measured by the sensor after increasing the particulate matter that flows into the capturing device through the control unit.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on and incorporates herein by reference Japanese Patent Application No. 2008-147973 filed on Jun. 5, 2008.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an exhaust emission control device for an internal combustion engine.

2. Description of Related Art

In a diesel engine, it is important to remove ‘particulate matter (PM)’ such as black smoke discharged from an engine. To this end, a diesel particulate filter (DPF) is often provided in an exhaust pipe.

By the DPF capturing PM, most of the PM in exhaust gas is removed. However, because the DPF is clogged if the PM continues depositing in the DPF, the DPF is recovered by removing the deposited PM through burning it by methods such as post injection when a deposit amount of PM reaches a certain level or above. The DPF may fail due to its long-term continuous use. The failure includes damage and melting loss because of excessive temperature rising during recovering.

When the DPF fails, the PM passes through the DPF so as to deteriorate emissions. Accordingly, a technology to detect failure of the DPF is needed. For instance, a technology to detect failure of the DPF using an amount of soot measured by a soot sensor provided on a downstream side of the DPF, is disclosed in JP2007-315275A. When the DPF fails, an amount of PM passing through the DPF is increased as compared to when the DPF is not at fault. Consequently, using a measurement value by the soot sensor on the downstream side of the DPF, failure is detected.

According to JP2007-315275A, when a deposit amount of PM in the DPF is small, failure detection of the DPF is not performed. This is because, as maintained in JP2007-315275A, it is difficult to distinguish between PM which has passed due to failure and PM which has passed even though the DPF is not at fault, since a PM capture rate by the DPF is originally low under circumstances where the PM deposit amount is small.

However, several problems may be pointed out in the point of view in JP2007-315275A. First of all, as described above, according to this document, the DPF failure detection is performed when the PM deposit amount is large. Because the PM capture rate is high when the PM deposit amount is large, the amount of PM passing through the DPF is small. Accordingly, the measurement value by the soot sensor is very small. Since the proportion of errors in the measurement value is high when the measurement value by the soot sensor is very small, failure of the DPF may not be detected with a high degree of accuracy.

Furthermore, when the PM deposit amount is large, the deposited PM sometimes captures newly inflowing PM even though the DPF is at fault. For this reason, influence of the DPF failure may hardly be reflected in the measurement value by the soot sensor on the downstream side. On the other hand, a possibility of the PM capture by the deposited PM is low when the PM deposit amount is small. Therefore, it seems that the influence of the DPF failure is sharply reflected in the measurement value by the soot sensor on the downstream side. Accordingly, contrary to JP2007-315275A, the case of the small PM deposit amount is more suitable to detect failure of the DPF.

Moreover, as described above, according to JP2007-315275A, the DPF failure detection is performed when the PM deposit amount is large. Consequently, the amount of PM passing through the DPF is small, so that the measurement value by the soot sensor may be very small. Hence, the DPF failure detection cannot be performed with high accuracy when sensitivity of the soot sensor is not high. Intentionally increasing an amount of PM flowing into the DPF when the amount of PM passing through the DPF is small and thus PM is poorly detected by the soot sensor, may be one idea. However, such a coping technique is not considered in JP2007-315275A.

In addition, if the DPF failure detection is not performed when the PM deposit amount is small as in JP2007-315275A, as a matter of course, a period during which the failure detection of the DPF is not performed arises. However, in order to promptly detect the failure of the DPF, it is desirable to avoid excessive prolongment of a blank period of the failure detection of the DPF.

For the above-described reasons, unlike JP2007-315275A, there is a need to detect failure of the DPF when the PM deposit amount is small (or with respect to any PM deposit amount). And, there is also a need to increase intentionally the amount of PM flowing into the DPF and to avoid the excessively prolonged blank period of the failure detection of the DPF, in order to accurately detect the failure of the DPF regardless of the sensitivity of the soot sensor.

SUMMARY OF THE INVENTION

The present invention addresses the above disadvantages. Thus, it is an objective of the present invention to provide an exhaust emission control device for an internal combustion engine for measuring an amount of PM passing through a DPF so as to detect failure of the DPF, more specifically, to provide an exhaust emission control device which detects the failure of the DPF promptly as well as accurately by detecting the failure of the DPF when an amount of deposited PM is small (or with respect to any PM deposit amount), by increasing intentionally an amount of PM flowing into the DPF, by avoiding an excessively prolonged blank period of the failure detection of the DPF, or otherwise.

To achieve the objective of the present invention, there is provided an exhaust emission control device for an internal combustion engine of a vehicle. The device includes a capturing device, a sensor, a control means, and a determination means. The capturing device is disposed in an exhaust passage of the engine and configured to capture particulate matter in exhaust gas flowing through the exhaust passage. The sensor is disposed in the exhaust passage on a downstream side of the capturing device in a flow direction of exhaust gas and configured to measure an amount of soot in the particulate matter. The control means is for increasing the particulate matter that flows into the capturing device. The determination means is for determining whether the capturing device is at fault based on the amount of soot measured by the sensor after increasing the particulate matter that flows into the capturing device through the control means.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with additional objectives, features and advantages thereof, will be best understood from the following description, the appended claims and the accompanying drawings in which:

FIG. 1 is a schematic view illustrating a configuration of an exhaust emission control device for an internal combustion engine according to first and second embodiments of the invention;

FIG. 2 is a flow chart illustrating procedures for processing of DPF failure determination according to the first embodiment of the invention;

FIG. 3 is a flow chart illustrating procedures for processing of DPF failure determination according to a second embodiment of the invention;

FIG. 4 is a graph illustrating a relationship between an amount of PM passing through a DPF and a flow of exhaust gas flowing into the DPF according to the first and second embodiments of the invention;

FIG. 5 is a graph illustrating a relationship between the amount of PM passing through the DPF and concentration of smoke flowing into the DPF according to the first and second embodiments of the invention;

FIG. 6 is a graph illustrating a relationship between the amount of PM passing through the DPF and a PM deposit amount in the DPF, and a setting of a threshold value according to the first and second embodiments of the invention; and

FIG. 7 is a graph illustrating a relationship between DPF differential pressure and the PM deposit amount according to the first and second embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention are described below with reference to the accompanying drawings.

First Embodiment

An example of an exhaust emission control device 1 shown in FIG. 1 is configured for a four-cylinder diesel engine 2 (hereinafter referred to simply as an engine), and includes an intake pipe 3, an exhaust pipe 4, and an EGR pipe 5. The engine 2 and the exhaust emission control device 1 may be installed in a vehicle.

Air is supplied to the engine 2 through the intake pipe 3, and exhaust gas is discharged into an exhaust pipe 4. An airflow meter 31 and an intake air throttle 32 are provided for the intake pipe 3. Intake volume is measured by the airflow meter 31. The intake volume is adjusted by increase and decrease of a degree of opening of the intake air throttle 32.

An injector 21 is provided for the engine 2 for supplying fuel into the inside of a cylinder. An engine rotational speed sensor 22 is provided for the engine 2 for measuring an engine rotational speed.

Exhaust gas recirculation (EGR) whereby exhaust gas is flowed back from the exhaust pipe 4 to the intake pipe 3 is performed through the EGR pipe 5. Combustion temperature in the engine 2 is limited so as to reduce an amount of NOx (nitrogen oxide) discharged by the exhaust gas recirculation. An EGR valve 51 is provided for the EGR pipe 5 for adjusting an amount of exhaust gas to be returned.

A DPF 6 is disposed in the exhaust pipe 4. Exhaust temperature sensors 61, 62 are arranged respectively on an inlet side and discharge side of the DPF 6 for measuring exhaust gas temperatures at each position. A differential pressure sensor 63 for measuring a front-back differential pressure (differential pressure or DPF differential pressure), which is a difference between exhaust pressures on the inlet side and discharge side of the DPF 6, is also provided.

For example, a structure whereby an entrance side and discharge side of a ‘honeycomb structure’ are alternately clogged may be used as a exemplary structure of the DPF 6. An oxidation catalyst DPF in which the oxidation catalyst is supported may be employed as the DPF 6. Particulate matter (PM) is contained in exhaust gas discharged while the engine 2 is in operation, and the PM is captured by the inside or surface of the DPF wall when exhaust gas passes through a DPF wall of the DPF 6 having the above-described structure.

The deposited PM is removed by burning it every time a deposit amount of the PM deposited in the DPF 6 becomes large enough, so as to recover the DPF 6. The DPF 6 may be recovered in the following manner. For instance, post injection is performed after main injection from the injector 21. Then, temperature of unburned fuel sent to the DPF 6 by the post injection rises through the action of the oxidation catalyst supported by the DPF 6 so as to burn the PM deposited in the DPF 6.

A soot sensor 64 is provided on a downstream side of the DPF 6 in the exhaust pipe 4. An amount of PM which has passed through the DPF 6 without being captured by the DPF 6 is measured by the soot sensor 64. An amount of soot alone (soot amount) in the PM may also be used as the amount of PM (PM amount). Also, a concentration of PM in exhaust gas or a concentration of soot in exhaust gas may also be used as the PM amount or the soot amount.

The exhaust emission control device 1 includes an electronic control unit (ECU) 7. The ECU 7 has a structure of a computer, and includes a central processing unit (CPU) which performs various operations, a random access memory (RAM) which is its work area, and a memory 71 which stores a variety of information. The fuel injection into the engine 2 by the injector 21, the adjustment of the degree of opening of the intake air throttle 32 and the EGR valve 51, and the like are controlled by the ECU 7. Measurement values by the airflow meter 31, the engine rotational speed sensor 22, the exhaust temperature sensors 61, 62, the differential pressure sensor 63, and the soot sensor 64 are sent to the ECU 7.

The ECU 7 also has a function of calculating a travel distance, for instance, by obtaining information from a speed sensor 80 for measuring a vehicle speed. The ECU 7 is provided with a timer 72, thereby having a function of measuring elapsed time. The ECU 7 indicates a variety of information on a display 81, thereby transmitting information to a driver of the vehicle.

In a first embodiment of the invention, processing of failure detection (failure determination) of the DPF 6 is performed based on the above configuration. Procedures for the DPF failure detection processing in FIG. 2 are described below. The procedures in FIG. 2 may be processed automatically by the ECU 7.

First, at S10, whether an exhaust gas flow or smoke concentration is equal to or larger than a predetermined value is determined. The ‘smoke concentration’ is a concentration of smoke discharged from the engine 2 in exhaust gas. The PM is contained in the smoke. In FIG. 2, their respective predetermined values are expressed as A1 and A2. If the exhaust gas flow or smoke concentration is equal to or larger than the predetermined value (S10: YES), control of the ECU 7 proceeds to S40. If both the exhaust gas flow and smoke concentration are smaller than the predetermined value (S10: NO), control proceeds to S20.

At S10, control may proceed to S40 if one of the exhaust gas flow and smoke concentration is equal to or larger than the predetermined value. Such a case may be suitable to perform more frequently the failure detection because conditions for execution of the DPF failure detection become milder. At S10, control may proceed to S40 if both the exhaust gas flow and smoke concentration are equal to or larger than the predetermined value. This case may be suitable when a need to avoid the failure detection under circumstances which are unsuitable for the failure detection is great, since the conditions for execution of the DPF failure detection become more strict. A method of calculating the exhaust gas flow is described in greater detail hereinafter. The concentration of smoke discharged from the engine 2 is considered to be determined by operating conditions of the engine 2 (load, engine rotational speed).

A functional relation (map) from the operating conditions of the engine 2 to the smoke concentration is stored in the memory 71 in advance, and the ECU 7 may obtain the smoke concentration based on the map and the operating conditions. The load among the operating conditions may be, for instance, a command value for a fuel injection amount from the injector 21. The engine rotational speed may be a measurement value of the engine rotational speed sensor 22.

Generally, if the exhaust gas flow and smoke concentration are too small, failure of the DPF 6 cannot be detected accurately because a measurement value of the soot sensor (described in greater detail hereinafter) is too small. The determination at S10 is a procedure for avoiding this. If the exhaust gas flow and smoke concentration are too small, control proceeds to S30 hereinafter described in detail to perform processing for increasing the amount of PM flowing into the DPF 6.

At S20, whether an elapsed time or travel distance since the processing of the previous failure determination of the DPF 6 is equal to or larger than a predetermined value is determined. In FIG. 2, their respective predetermined values are expressed as A3 and A4. If the elapsed time or travel distance since the processing of the previous failure determination of the DPF 6 is equal to or larger than the predetermined value (S20: YES), control of the ECU 7 proceeds to S30. If they are smaller than the predetermined value (S20: NO), the processing in FIG. 2 is ended. The elapsed time since the processing of the previous failure determination of the DPF 6 may be measured by the timer 72. The travel distance since the processing of the previous failure determination of the DPF 6 may be calculated at the ECU 7 based on a measurement value of the speed sensor 80.

S20 is processing which is designed to proceed to failure determination (at S70 hereinafter described in detail) without fail if the elapsed time or travel distance since the previous failure detection is equal to or larger than the predetermined value. Accordingly, if the processing in FIG. 2 is performed at intervals of a certain period, the elapsed time or travel distance from the previous failure determination to the following failure determination falls within a certain predetermined value without fail. The certain predetermined value may be a value as a result of the addition of a value determined by the certain period to A3 or A4. As a result, failure is promptly detected when it occurs, without continuing to discharge the PM into the outside of the vehicle by leaving the failure unattended.

At S20, control may proceed to S30 if one of the elapsed time and travel distance since the processing of the previous failure determination of the DPF 6 is equal to or larger than the predetermined value. Such a case may be suitable when extension of a blank period of the failure detection is undesirable since the conditions for execution of the DPF failure detection become milder. Also, at S20, control may proceed to S30 if both the elapsed time and travel distance since the processing of the previous failure determination of the DPF 6 are equal to or larger than the predetermined value. Such a case may be suitable to avoid unnecessary failure detection because the conditions for execution of the DPF failure detection become more strict.

Accordingly, a period from the previous determination to the following determination of the failure of the capturing device 6 is set to be not too long as compared to a predetermined range. Therefore, a problem that a blank period during which failure detection of the capturing device 6 is not performed is too long to detect promptly the failure of the capturing device 6 is avoided.

Next, at S30, the ECU 7 performs the processing for increasing the amount of PM flowing into the DPF 6. A method of increasing the amount of PM flowing into the DPF 6 may be as follows. First, as one method, there is a method of increasing a flow of exhaust gas flowing into the DPF 6. The exhaust gas flow may be an exhaust gas flow per unit time.

A relationship between the flow of exhaust gas flowing into the DPF 6 and an amount of PM passing through the DPF 6 is illustrated in FIG. 4. As shown in FIG. 4, more PM tends to pass through the DPF 6 without being captured by the DPF 6 as the flow of exhaust gas flowing into the DPF 6 becomes larger. Accordingly, by utilizing this tendency, at S30, by increasing the flow of exhaust gas flowing into the DPF 6, the amount of PM passing through the DPF 6 is increased. A method of increasing the flow of exhaust gas flowing into the DPF 6 may be, for example, to increase the opening degree of the intake air throttle 32 by decreasing the opening degree of the EGR valve 51. Accordingly, by decreasing the degree of opening of the exhaust gas recirculating valve 51 and by increasing the degree of opening of the intake valve 32, the flow of exhaust gas flowing into the capturing device 6 is reliably increased. Thus, because the flow of exhaust gas is increased, the particulate matter flowing into the capturing device 6 is increased. Therefore, the amount of soot passing through the capturing device 6 is also increased, so that the measurement value by the sensor 64 is prevented from having a very small value. As a result, reliability of the measurement value of the sensor 64 improves. Consequently, using the measurement value of the reliable sensor 64, failure of the capturing device 6 is determined accurately.

Another method of increasing the amount of PM flowing into the DPF 6 at S30 may be to increase concentration of smoke in exhaust gas discharged from the engine 2. A relationship between the smoke concentration in exhaust gas flowing into the DPF 6 and the amount of PM passing through the DPF 6 is shown in FIG. 5. As shown in FIG. 5, since the amount of PM flowing into the DPF 6 increases as the smoke concentration in exhaust gas flowing into the DPF 6 becomes higher, the amount of PM passing through the DPF 6 also increases.

Accordingly, by utilizing this tendency, at S30, by increasing the smoke concentration in exhaust gas flowing into the DPF 6, the amount of PM passing through the DPF 6 may be increased. A method of increasing the smoke concentration in exhaust gas flowing into the DPF 6 is, for example, to adjust by the ECU 7 the injection pressure, injection amount, and injection timing of fuel from the injector 21 so as to increase the smoke concentration. The map may be used, by making the memory 71 store in advance as a map how much the smoke concentration comes to according to with how much injection pressure and injection amount and which injection timing fuel is injected from the injector 21. Accordingly, by adjusting at least one of the fuel injection pressure, fuel injection amount, and fuel injection timing of the internal combustion engine 2, the smoke discharged from the internal combustion engine 2 is reliably increased. Thus, by increasing the smoke discharged from the internal combustion engine 2, the particulate matter flowing into the capturing device 6 is increased. Hence, the amount of soot passing through the capturing device 6 is also increased, so that the measurement value by the sensor 64 is prevented from having a very small value. As a result, failure of the capturing device 6 is determined accurately using the measurement value of the reliable sensor 64. After the processing at S30 is ended, control of the ECU 7 proceeds to S40.

At S40, the ECU 7 measures an amount of soot which has passed through the DPF 6. This may be measured by the soot sensor 64. At S50, the ECU 7 estimates a deposit amount of PM in the DPF 6. A method of estimating the deposit amount of PM in the DPF 6 is described in greater detail hereinafter.

At S60, the ECU 7 calculates a threshold value (failure determination threshold value) used in the failure determination of the DPF 6 at S70, which is hereinafter described in detail. A method of calculating the threshold value is illustrated in FIG. 6. FIG. 6 is a graph with the deposit amount of PM in the DPF 6 and an amount of PM passing through the DPF 6 respectively indicated along its horizontal axis and vertical axis. In FIG. 6, a continuous line illustrates a relationship between the PM deposit amount when the DPF 6 is not at fault and the amount of PM passing through the DPF 6.

The continuous line in FIG. 6 may express actual measurement values obtained in experiment or the like in advance with respect to the DPF 6 which is actually used in the configuration of FIG. 1. As shown in FIG. 6, as the PM deposit amount in the DPF 6 is smaller, a capture rate of the DPF 6 is lower, so that the amount of PM passing through the DPF 6 tends to increase. Values obtained as a result of the addition of a predetermined amount to this continuous line are indicated by a short dashes line. At S60, the threshold value may be calculated using this short dashes line.

More specifically, at S60, on the short dashes line in FIG. 6, a value on its vertical axis with the PM deposit amount obtained at S50 plotted on its horizontal axis, is calculated as the threshold value. Because the threshold value is calculated using the short dashes line in FIG. 6, an appropriate threshold value in accordance with the PM deposit amount is set. Characteristics of FIG. 6 may be in advance stored in the memory 71.

FIG. 6 may be a graph provided that the amount of PM flowing into the DPF 6 per unit time or the flow of exhaust gas is constant. Then, FIG. 6 may be used at S60 after its three-dimensional modification in the following manner. More specifically, the continuous line in FIG. 6 is modified into a functional relation (map) from the amount of PM flowing into the DPF 6 per unit time or the exhaust gas flow and the PM deposit amount in the DPF 6 to the amount of PM passing through the DPF 6. Then, for instance, similar to S10, the exhaust gas flow and smoke concentration are obtained, and the amount of PM flowing into the DPF 6 per unit time is calculated based on these two. Also, as described above, the PM deposit amount in the DPF 6 is estimated at S50. The amount of PM passing through the DPF 6 is obtained from these figures and the above three-dimensional map, and a value obtained as a result of the addition of a predetermined value to this is calculated as the threshold value at S60.

The amount of PM passing through the DPF 6 may mean a concentration of PM in exhaust gas passing through the DPF 6. The amount of PM flowing into the DPF 6 per unit time may also be, for example, a value which is exponentiated provided that a reference value is 1.

Accordingly, it is determined that the capturing device 6 is at fault if the measurement value of the amount of soot by the sensor 64 is larger than a threshold value, and the threshold value is set based on the amount of particulate matter deposited in the capturing device 6. Therefore, failure detection is able to be performed on the capturing device 6 with respect to a small deposit amount or any deposit amount as well as a large amount of particulate matter in the capturing device 6. Using the threshold value which is adjusted appropriately in accordance with the amount of particulate matter deposited in the capturing device 6, failure of the capturing device 6 is accurately detected.

Next, at S70, the ECU 7 determines whether the DPF 6 has failed. The soot sensor measurement value obtained at S40 and the threshold value calculated at S60 are used in the determination of failure. If the soot sensor measurement value is equal to or larger than the threshold value (S70: YES), control proceeds to S80. If the soot sensor measurement value is smaller than the threshold value (S70: NO), control proceeds to S90.

When control proceeds to S80, i.e., when the soot sensor measurement value is equal to or larger than the threshold value, the ECU 7 determines that the DPF 6 has failed because the amount of soot which has passed through the DPF 6 is too large. When control proceeds to S90, i.e., when the soot sensor measurement value is smaller than the threshold value, the ECU 7 determines that the DPF 6 is not at fault.

After the processing at S80 is ended, control of the ECU 7 proceeds to S100. At S100, in view of the determination of the DPF 6 to be failure at S80, the ECU 7 indicates to the driver that the DPF 6 has failed on the display 81. Consequently, the driver obtains information about the failure, so that the driver can promptly go on to procedures such as repair of the DPF 6. These are the procedures for the processing in FIG. 2.

Accordingly, in determining the failure of the capturing device 6 that is disposed in the exhaust passage 4 for capturing particulate matter, the exhaust emission control device 1 for the internal combustion engine 2 according to the first embodiment measures an amount of soot of particulate matter which has passed through the capturing device 6 so as to determine the failure of the capturing device 6 after increasing particulate matter flowing into the capturing device 6. Therefore, the amount of soot passing through the capturing device 6 is also increased due to the deliberately-increased particulate matter, so that the measurement value by the sensor 64 is prevented from having a very small value. As a result, reliability of the measurement value of the sensor 64 improves. Consequently, using the measurement value of the reliable sensor 64, failure of the capturing device 6 is determined accurately.

Second Embodiment

Procedures for the processing of failure detection (failure determination) of a DPF 6 are described below. In a second embodiment of the invention, if a PM deposit amount in the DPF 6 is equal to or smaller than a predetermined value, the failure detection of the DPF 6 using a soot sensor 64 is performed after increasing an amount of PM flowing into the DPF 6. Accordingly, as described above, failure of the DPF 6 is detected accurately. The procedures in FIG. 3 may be processed automatically by an ECU 7. Configuration of units in the second embodiment may be the same as FIG. 1.

First, the ECU 7 estimates the PM deposit amount in the DPF 6 at procedure S110. A method of estimating the deposit amount of PM in the DPF 6 is described in greater detail hereinafter. Next, at S120, the ECU 7 determines whether the PM deposit amount is equal to or smaller than a predetermined value. In FIG. 3, this predetermined value is expressed as A5. If the PM deposit amount is equal to or smaller than the predetermined value (S120: YES), control of the ECU 7 proceeds to S130. If the PM deposit amount is larger than the predetermined value (S120: NO), the ECU 7 ends the processing in FIG. 3.

At S130, the ECU 7 performs the processing for increasing the amount of PM flowing into the DPF 6. The processing at S130 may be performed similar to the above S30. At S140 that follows, the ECU 7 obtains a measurement value of a soot sensor 64. Then, at S150, the ECU 7 calculates a threshold value used in the failure determination of the DPF 6. The calculation of the threshold value at S150 may be performed similar to the calculation of the threshold value at S60 in FIG. 2.

Next, at S160, the ECU 7 determines whether the DPF 6 has failed, The soot sensor measurement value obtained at S140 and the threshold value calculated at S150 are used in the determination of failure. If the soot sensor measurement value is equal to or larger than the threshold value (S160: YES), control proceeds to S170. If the soot sensor measurement value is smaller than the threshold value (S160: NO), control proceeds to S180.

When control proceeds to S170, i.e., when the soot sensor measurement value is equal to or larger than the threshold value, the ECU 7 determines that the DPF 6 has failed because the amount of soot which has passed through the DPF 6 is too large. When control proceeds to S180, i.e., when the soot sensor measurement value is smaller than the threshold value, the ECU 7 determines that the DPF 6 is not at fault.

After the processing at S170 is ended, control of the ECU 7 proceeds to S190. At S190, in view of the determination of the DPF 6 to be failure at S170, the ECU 7 indicates to the driver that the DPF 6 has failed on a display 81. Consequently, the driver obtains information about the failure, so that the driver can promptly go on to procedures such as repair of the DPF 6. These are the procedures for the processing in FIG. 3.

Accordingly, the exhaust emission control device 1 for the internal combustion engine 2 according to the second embodiment estimates an amount of particulate matter deposited in the capturing device 6, and determines whether the capturing device 6 has failed if an estimated value of the deposit amount is smaller than a predetermined deposit amount. Therefore, a problem that, the failure of the capturing device 6 is difficult to determine by the amount of soot passing through the capturing device 6 despite the failure of the capturing device 6 by the deposited particulate matter capturing particulate matter newly flowing into the capturing device 6 when the deposited amount of particulate matter is larger, is avoided. Thus, by taking advantage of prominent reflection of the influence of failure of the capturing device 6 in the amount of soot passing through the capturing device 6 since the deposited amount of particulate matter is small, delicate failure of the capturing device 6 is detected accurately.

A method of the estimation of the deposit amount of PM in the DPF 6, which is performed at S50 in FIG. 2 and S110 in FIG. 3, is described below. A relationship between the PM deposit amount and the DPF differential pressure is generally expressed as (or approximates) a relationship illustrated in FIG. 7. More specifically, as the operation of the internal combustion engine continues so that the deposition of PM in the DPF develops, a point indicating the PM deposit amount and the DPF differential pressure moves from an initial point 100 to the upper right in FIG. 7 along a first characteristic line 110 (characteristic line). Furthermore, after the point reaches a transition point 120, the point moves to the upper right in FIG. 7 along a second characteristic line 130 (characteristic line).

The first characteristic line 110 corresponds to a stage where PM is deposited in pores on a filter wall of the DPF, and the second characteristic line 130 corresponds to a stage where PM is deposited on a wall surface of the filter wall. In the case of the deposition of PM in the filter wall, a degree of newly narrowing a passage for exhaust gas is higher than in the case of deposition on the wall surface, and accordingly a differential pressure value is increased. As a result, as shown in FIG. 7, the first characteristic line 110 has a larger inclination than the second characteristic line 130. The inclination may be a ratio between increment of the DPF differential pressure and increment of the PM deposit amount.

Given that when a point 140 is reached, the PM deposit amount is determined to be surplus, so that DPF recovery is started, the subsequent PM deposit amount and DPF differential pressure shift as indicated by a short dashes line in FIG. 7. More specifically, the PM deposit amount and the DPF differential pressure decrease along a short dashes line 150 first, and decrease along a short dashes line 170 after a transition point 160. Finally, they return to the initial point 100.

The short dashes line 150 expresses a stage where PM deposited in pores on the filter wall is burning, and thus, the short dashes line 150 has the same inclination as the first characteristic line 110. The short dashes line 170 expresses a stage where PM deposited on the wall surface of the filter wall is burning, and thus the short dashes line 170 has the same inclination as the second characteristic line 130. As described above, according to characteristics of a parallelogram illustrated in FIG. 7 (or characteristics approximating a parallelogram), the PM deposit amount and the DPF differential pressure when PM is deposited and when PM is burning vary. Characteristics illustrated in FIG. 7 are obtained beforehand, and stored in the memory 71. Then, the PM deposit amount may be obtained based on the DPF differential pressure value measured.

Next, a method of calculating the flow of exhaust gas used at S10 in FIG. 2 is described below. The flow may be a volumetric flow per unit time. A mass flow rate of intake air per unit time measured by the airflow meter 31 is converted into a volumetric flow of exhaust gas. The calculation of the volumetric flow of exhaust gas is performed in accordance with the following equation (E1) (when a downstream side of the DPF has atmospheric pressure). V (m³/sec) expresses a volumetric flow of exhaust gas per unit time, G (g/sec) expresses a mass flow rate of intake air per unit time, Tdpf (K) expresses DPF temperature, P0 (kPa) expresses atmospheric pressure, ΔP (kPa) expresses DPF differential pressure, and Q (cc/sec) expresses a fuel injection amount per unit time.

$\begin{matrix} {{V\left( {m^{3}\text{/}\sec} \right)} = {\left\lbrack \frac{G\left( {g\text{/}\sec} \right)}{28.8\mspace{11mu} \left( {g\text{/}{mol}} \right)} \right\rbrack \times 22.4 \times 10^{- 3}\mspace{11mu} \left( {m^{3}\text{/}{mol}} \right) \times \left\lbrack \frac{{Tdpf}(K)}{273(K)} \right\rbrack \times {\quad{\left\lbrack \frac{P\; 0\left( {k\; {Pa}} \right)}{\left( {{P\; 0({kPa})} + {\Delta \; {P({kPa})}}} \right)} \right\rbrack + {\frac{Q\left( {{cm}^{3}\text{/}\sec} \right)}{207.3\mspace{11mu} \left( {g\text{/}{mol}} \right)} \times 0.84\mspace{11mu} \left( {g\text{/}{cm}^{3}} \right) \times 6.75 \times 22.4 \times 10^{- 3}\mspace{11mu} \left( {m^{3}\text{/}{mol}} \right) \times \left\lbrack \frac{P\; 0({kPa})}{\left( {{P\; 0({kPa})} + {\Delta \; {P({kPa})}}} \right)} \right\rbrack}}}}} & ({E1}) \end{matrix}$

A first term on a right hand side of the equation (E1) expresses a conversion of a mass flow rate of intake air into a volumetric flow, and a second term expresses an increased part from intake air to exhaust gas due to the combustion of injected fuel, In the second term, 0.84 (g/cc) expresses exemplary liquid density of light oil. 22.4×10⁻³ (m³/mol) expresses volume of ideal gas per mol at 0 (zero) degrees Celsius and 1 atmosphere (atm). 6.75 expresses a rate of increase of the number of moles of exhaust gas with respect to 1 (mol) of the fuel injection amount.

The increase rate (6.75) is obtained in the following manner. Composition of light oil is typically expressed as C₁₅H_(27.3) (molecular weight 207.3), and combustion is expressed in the following reaction formula (E2). Accordingly, exhaust gas has 6.75 (=(15+13.5)−21.75) times as large number of moles as the fuel injection amount of 1 (mol).

C₁₅H_(27.3)+21.75O₂→15CO₂+13.5H₂O  (E2)

Fuel is injected only with predetermined injection timing determined by the ECU 7, and it presents intermittent injection. A fuel injection amount Q in the equation (E1) is an average fuel injection amount including a non-injection period.

The mass flow rate of intake air per unit time G (g/sec) may be measured by the airflow meter 31. The DPF temperature Tdpf (K) may be measured by the exhaust temperature sensors 61, 62. The DPF front-back differential pressure ΔP (kPa) may be measured by the differential pressure sensor 63. The command value for the injection amount given to the injector 21 by the ECU 7 may be used for the fuel injection amount per unit time Q (cc/sec).

The DPF temperature Tdpf (K) may be one of measurement values by the exhaust temperature sensors 61, 62, or may bean average value of both the measurement values. Alternatively, a model for estimating internal temperature of the DPF 6 from measurement value(s) by one of the exhaust temperature sensors 61, 62 or by both the sensors 61, 62 is obtained beforehand, and is stored in the memory 71. Then, the DPF temperature Tdpf (K) may be estimated using this model. The above are the method of calculating the flow of exhaust gas.

In the above embodiments, the procedures of S30, S130 and the ECU 7 constitute a ‘control means’, the procedures of S70, S160 and the ECU 7 constitute a ‘determination means’, the procedures of S30, S130, the ECU 7, the intake air throttle 32, and the EGR valve 51 constitute a ‘first increasing means’ and an ‘opening degree adjustment means’, the procedures of S30, S130, the ECU 7, and the injector 21 constitute a ‘second increasing means’ and an ‘injection adjustment means’, the procedures of S50, S110 and the ECU 7 constitute an ‘estimation means’, the procedures of S60, S150 and the ECU 7 constitute a ‘threshold value setting means’, and the procedure of S20 and the ECU 7 constitute a ‘determination performing means’. When a lean burn gasoline engine, instead of a diesel engine, is used for the engine 2 in the above embodiments, similar effects to the above description are produced.

Additional advantages and modifications will readily occur to those skilled in the art. The invention in its broader terms is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described. 

1. An exhaust emission control device for an internal combustion engine, comprising: a capturing device disposed in an exhaust passage of the engine and configured to capture particulate matter in exhaust gas flowing through the exhaust passage; a sensor disposed in the exhaust passage on a downstream side of the capturing device in a flow direction of exhaust gas and configured to measure an amount of soot in the particulate matter; a control means for increasing the particulate matter that flows into the capturing device; and a determination means for determining whether the capturing device is at fault based on the amount of soot measured by the sensor after increasing the particulate matter that flows into the capturing device through the control means.
 2. The exhaust emission control device according to claim 1, wherein the control means includes a first increasing means for increasing a flow of exhaust gas that flows into the capturing device.
 3. The exhaust emission control device according to claim 2, further comprising: an exhaust gas recirculating passage through which exhaust gas recirculates from the exhaust passage to an intake passage of the engine; an exhaust gas recirculating valve disposed in the exhaust gas recirculating passage; and an intake valve disposed in the intake passage, wherein the first increasing means includes an opening degree adjustment means for decreasing a degree of opening of the exhaust gas recirculating valve and for increasing a degree of opening of the intake valve.
 4. The exhaust emission control device according to claim 1, wherein the control means includes a second increasing means for increasing smoke in exhaust gas discharged from the engine.
 5. The exhaust emission control device according to claim 4, wherein the second increasing means includes an injection adjustment means for adjusting at least one of fuel injection pressure, fuel injection amount, and fuel injection timing of the engine.
 6. The exhaust emission control device according to claim 1, further comprising an estimation means for estimating an amount of the particulate matter deposited in the capturing device, wherein the determination means determines whether the capturing device is at fault when the amount of the deposited particulate matter that is estimated by the estimation means is smaller than a predetermined value.
 7. The exhaust emission control device according to claim 1, wherein the determination means determines that the capturing device is at fault when the amount of soot measured by the sensor is larger than a threshold value, the device further comprising a threshold value setting means for setting the threshold value using an amount of the particulate matter deposited in the capturing device.
 8. The exhaust emission control device according to claim 1, further comprising a determination performing means for performing a following failure determination on the capturing device through the determination means before an elapsed time and/or a travel distance of the vehicle since a previous failure determination of the capturing device through the determination means exceeds a predetermined value. 